1. Field of the Invention
The present invention relates to a driving method of a light emitting device having a light emitting element on the substrate. Especially, it relates to the driving method of a light emitting device in which an operation of the light emitting element is controlled by using a semiconductor device (a device using a semiconductor thin film).
2. Description of the Related Art
Recently, techniques for forming a TFT on a substrate has been greatly advanced, and much developments have been made to apply those techniques to an active-matrix type display device. In particular, a TFT employing a poly silicon film can operate at a higher speed since a field effect mobility (simply referred to as the mobility) thereof is larger than that of a TFT employing the conventional amorphous silicon film. Thus, it becomes possible to control pixels by means of a driver circuit formed on the same substrate as the pixels. Such the pixels were conventionally controlled by means of a driver circuit provided at the outside of the substrate.
The active-matrix type display device as mentioned above can exhibit various advantages such as a reduced fabricating cost, miniaturization of the display device, an increased fabricating yield, a reduced throughput or the like, by providing various circuits and devices on the identical substrate.
Furthermore, researches of an active-matrix type light emitting device having light emitting elements have been actively conducted. Such a light emitting device is also referred to as an Organic EL Display (OELD) or an Organic Light Emitting Diode (OLED).
In the present specification, the EL element that is a light emitting element formed in the pixel of an organic light emitting device is described as an example of a typical light emitting element.
Unlike a liquid crystal display device, the light emitting device is of the self-emission type. An EL element has a structure that the light emitting layer is placed between a cathode and an anode, but a light emitting layer usually has a laminated structure. Typical examples therefore include a laminated structure of “a hole transportation layer/an organic compound layer/an electron transportation layer” proposed by Tang et al. of Eastman Kodak Co. This structure has a high luminous efficiency, and most of light emitting devices about which research and development activities are currently being progressed employ this structure.
Alternatively, a laminated structure in which a hole injection layer/a hole transportation layer/an organic compound layer/an electron transportation layer, or a hole injection layer/a hole transportation layer/an organic compound layer/an electron transportation layer/an electron injection layer are formed in these orders may be used. Furthermore, fluorescent dyes or the like may be doped into the organic compound layer.
In the present specification, all of the layers to be disposed between the cathode and the anode are collectively referred to as the light emitting layer. Accordingly, all of the above-mentioned layers such as the hole injection layer, the hole transportation layer, the organic compound layer, the electron transportation layer, the electron injection layer or the like are included in the light emitting layer.
A predetermined voltage is applied to the light emitting layer made of the above-mentioned structure via the pair of electrodes, and thus recombination of carriers occurs in the light emitting layer, thereby resulting in light emission. In the present specification, when the EL element emits light, the EL element is expressed as being driven.
3. Problems to be Resolved by the Invention
Known gray scale display methods for light emitting devices are divided into analog methods and digital time division methods.
Analog gray scale display of a light emitting device is described with reference to FIGS. 1 and 2.
FIG. 1 shows the structure of a pixel portion 1800 of a light emitting device. The pixel portion is composed of (x x y) (x and y are natural numbers equal to or larger than 1) pixels arranged to form a matrix pattern. Gate signals are input to y gate signal lines (G1 to Gy), which are each connected to a gate electrode of a switching TFT 1801 of each pixel. The switching TFT 1801 of each pixel has a source region and a drain region one of which is connected to one of x source signal lines (S1 to Sx) (also called data signal lines) to which analog video signals are input and the other of which is connected to a gate electrode of a driving TFT 1804 of each pixel.
The driving TFT 1804 of each pixel has a source region and a drain region one of which is connected to a power supply line 1810 and the other of which is connected to an EL element 1806. The power supply line 1810 is kept at a certain electric potential and this electric potential is denoted by VD.
A capacitor 1808 may be provided between the gate electrode of the driving TFT 1804 and the power supply line 1810 to serve as a storage capacitor for holding the gate-source voltage of the driving TFT 1804.
The EL element 1806 is composed of an anode, a cathode, and a light emitting layer that is placed between the anode and the cathode. When the anode is connected to the source region or drain region of the driving TFT 1804, the cathode is connected to an opposite electrode 1809. On the other hand, when the cathode is connected to the source region or drain region of the driving TFT 1804, the opposite electrode 1809 is connected to the anode.
Though not shown in FIG. 1, the opposite electrode 1809 of each pixel is connected so as to have the same electric potential, which is denoted by VC.
FIG. 2 is a timing chart of when the light emitting device is driven by an analog method. One frame period (F) is a period necessary to write one screen of video signals and display an image. A period in which a gate signal line on one row is selected is called one line period (L). Since the light emitting device of FIG. 1 has y gate signal lines, one frame period has y line periods (L1 to Ly). A period from the end of selection of the gate signal line on the last row in a frame period until the start of selection of the gate signal line on the first row in the next frame period is called a vertical retrace period.
In a usual light emitting device, 60 or more frame periods are provided in one second to display 60 or more images per second. If the number of images displayed per second is less than 60, flickering of images is recognizable to the eye.
As the number of gate signal lines, y, becomes larger, line periods in one frame period are increased in number and the driving circuit has to be operated at higher frequency.
Next, how the analog drive light emitting device shown in FIG. 1 operates will be described referring to FIG. 2.
In Line Period 1 (L1), a selection signal is input from a gate signal line driving circuit to the gate signal line G1. Then, analog data signals are input to the source signal lines (S1 to Sx) in order.
The selection signal turns every switching TFT 1801 that is connected to the gate signal line G1 ON. Accordingly, the analog video signals input to the source signal lines (S1 to Sx) are input to the gate electrode of the driving TFT 1804 through the switching TFT 1801.
With the switching TFT 1801 turned ON, the electric potential of an analog video signal input into the pixel changes the electric potential of the gate electrode of the driving TFT 1804. At this point, the drain current is determined uniquely from the gate-source voltage in accordance with the voltage-current characteristic of the driving TFT 1804. A desired drain current is thus supplied to the EL element 1806, which emits light at a luminance according to the amount of the drain current.
The operation described above is repeated until inputting of analog video signals to source signal lines (S1 to Sx) is completed to end Line Period 1 (L1). Alternatively, one line period may be determined consisingt of a period necessary to complete inputting of analog video signals to the source signal lines (S1 to Sx) and a horizontal retrace period. After Line Period 1 (L1), Line Period 2 (L2) is started and a selection signal is input to the gate signal line G2. Then, similar to Line Period 1 (L1), analog video signals are input to the source signal lines (S1 to Sx) in order.
When every gate signal line (every one of G1 to Gy) has received a selection signal, all the line periods (L1 to Ly) are now finished. Finishing all the line periods (L1 to Ly) means the end of one frame period. During one frame period, every pixel is used to form an image for display. Alternative definition of one frame period is all the line periods (L1 to Ly) plus a vertical retrace period.
The electric potential VD of the power supply line 1810 and the electric potential VC of the opposite electrode of each pixel are set to levels that allow the light emitting element to carry out the above operation normally.
As described, analog data signals control the light emission luminance of EL elements for gray scale display. This is a driving method called an analog gray scale display method, and uses an electric potential difference of analog video signals to display an image in gray scales.
If the Id-Vg characteristic of the driving TFT is fluctuated among pixels, it is impossible to output the same drain current even when the same gate-source voltage is applied to the driving TFT of each pixel. Then the slightest fluctuation in Id-Vg characteristic causes EL elements of adjacent pixels to emit light in different amounts from one another even though signals of the same voltage are input to the pixels.
Analog gray scale display is thus very responsive to characteristic fluctuation among TFTs and this is an obstacle for a light emitting device to display an image in increased gray scales.
Described next are a technique disclosed in Japanese Laid-Open Publication No. 2001-5426 A as gray scale display by a digital time division method and its problems.
In order to increase the number of gray scales without changing the length of one frame period, more sub-frame periods have to be provided in one frame period. Therefore, it is necessary to operate the circuit for sending signals to pixels at higher speed. This results in an increase in power consumption. Also, an increase in number of address (writing) periods (Ta) leads to reduction in proportion of display periods to the entire length of one frame period (duty ratio). If the sum of sustain (lighting) periods (Ts) in one frame period amounts to half the one frame period, namely, if the duty ratio is 50%, the luminance in this case is half the luminance of when the duty ratio is 100%. To obtain the same level of luminance as when the duty ratio is 100%, the luminance at which an EL element emits light in a sustain (lighting) period, namely, instantaneous luminance, has to be doubled. This means that an EL element has to receive a doubled amount of current.